A widely used design for thermionic electron sources includes a plurality of RF acceleration structures, for example RF cavities. Thermionic electron sources, such as RF guns, are capable of providing high current electron beams and excellent emittance properties.
One limitation of RF electron sources that employ thermionic emitters is the heating of the emitter that occurs due to back-bombardment. When thermionic emitters are used with RF structures, there is a general incompatibility between the timing of a nominally DC emitter with the rapid varying temporal properties of the RF structure. One of the primary consequences is that, unless carefully designed, the energy of electrons that are directed back at the cathode can produce significant cathode heating due to this back-bombardment of the electrons.
As the pulse width, duty factor, and RF electric field of the extraction cavity are increased, the above-described cathode heating can quickly provide more cathode heating than the heater control. This results in both cathode damage, which can reduce lifetime, and control instability, which can disrupt the electron beam.